Pneumatic spring provided with a level measuring device

ABSTRACT

An air spring ( 2 ) essentially includes two end elements ( 4, 6 ) and an electrically conductive rolling-lobe flexible member which is pressure-tightly arranged therebetween and is made of a flexible elastomeric electroconductive material ( 14 ). A reinforcement ( 16 ) formed by two cord fiber plies ( 16   a,    16   b ), which are made of fibers ( 18 ), are vulcanized into the flexible member. In order to determine the height (h) of the spring, preferably, the two cord fabric plies ( 16   a,    16   b ) are provided with a number of highly conductive fibers ( 18   a ) which are arranged at the beginning and the end of each ply in a parallel direction to each other, thereby forming two conductive strips ( 24   a,    24   b ). The conductive strips ( 24   a,    24   b ) are disposed oppositely to each other and are electrically connected at the end of the rolling-lobe flexible member ( 8 ) in such a way that a conductive loop ( 24 ) is formed and each strip is used as an element in a branch for an alternating-current measuring bridge ( 28 ). The filaments ( 20 ) of individual fibers ( 18 ) or only certain filaments ( 20   a ) of the fibers ( 18 ) can be electrically conductive. In addition to determining the height (h) of the spring in a motor vehicle, the inventive device can also be used for determining the air pressure in the flexible member of the air spring ( 2 ), the temperature (T) of the flexible member walls and other measurement quantities.

RELATED APPLICATION

This application is the national stage of PCT/EP 2004/004495, filed Apr.28, 2004, designating the United States and claiming priority fromGerman patent application no. 103 25 624.5, filed Jun. 6, 2003, theentire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to an air spring having a level measuring unitsuch as known, for example, from the publications DE 100 17 562 C1, DE40 35 784 A1 and DE 44 13 559 A1.

BACKGROUND OF THE INVENTION

In each of the above examples, the air spring comprises essentially twovariably mutually spaced end members, namely, a cover and a roll-offpiston and a flexible member clamped pressure tight therebetween,especially, a rolling-lobe flexible member.

In publication DE 100 25 631 A1, a method is described wherein theheight of the spring is determined by means of the high frequency hollowspace resonance. The flexible member must have good conductivity so thatthe flexible member performs as an electromagnetic hollow spaceresonator. This can, for example, be achieved in that thereinforcements, which are introduced into the flexible member, areelectrically conductive.

This publication emphasizes details of the measuring electronics.Details as to the configuration of the electrically conductivereinforcements are not disclosed.

According to DE 100 17 562 C1, the measurement of height takes placewith the aide of two coils one of which is mounted axially secure withinthe air spring interior space and the other one of the coils is mountedbetween the cover and the roll-off piston so as to be changeable inlength. A level dependent measurement signal results because of thechange of the height position of the air spring as well as because ofthe compression operation. The length-changeable coil can be an integralcomponent of the flexible member, that is, of the wall. This coil iseither pressed onto the surface of the flexible member facing inwardlyor is glued or is worked directly between the layers.

An application of the coil of this kind on or in the wall of theflexible member requires an additional work step in the production ofthe flexible member or in the production of the air spring. Problems candevelop with the flexibility of the wall of the flexible member(harshness effect) because the coil is not mounted in the plane of thefabric ply or plies.

The flexible members of the air springs, described in publications DE 4035 784 A1 and DE 44 13 559 A1, likewise show measurement fibers workedinto the wall.

It is, however, the case that according to DE 40 35 784 A1, electricalconductors are worked into the wall of the flexible member in the formof a coil or diagonally. Here, the conductor paths are configured as acoil to be changeable in length with the coil being applied to a latexmonofil. The incorporation of latex monofil fibers, which are providedwith electrically conducting coils, into the wall of the flexible memberis likewise associated with additional work complexity in themanufacture.

According to DE 44 13 559 A1, the electrically conductive measuringfibers, which are integrated into the wall of the flexible member, arecharacterized by running parallel to the fiber direction of a fabric plyand in the longitudinal direction of the flexible member from oneflexible member end to the other. Because of the position and thearrangement of these conductor paths, their inductivity changes with thespring height because of the spring operation.

The fibers, which are to be introduced into the wall of the flexiblemember, comprise, for example, copper strands which must be introducedinto the wall of the flexible member in addition to the textile fabricplies or in lieu of individual fibers of the fabric plies. If the copperfibers are not arranged in the plane of the textile reinforcement, thenthere results overall a stiffening of the wall of the flexible memberand the consequences are a pronounced harshness effect. If the copperfibers are in the plane of the textile reinforcement, then there resultsan inhomogeneous expansion of the wall of the flexible member duringloading because of the different expansion characteristics of the copperstrands and textile cords whereby the service life of the flexiblemember is affected.

SUMMARY OF THE INVENTION

The task of the invention comprises providing a wall of the flexiblemember, which is provided with electrically conductive measuring fibersfor an air spring which does not exhibit the disadvantages listed fromthe state of the art.

The electrically conductive flexible member wall should functionespecially as measuring means to determine the spring height.

SOLUTION AND ADVANTAGES

The essential essence of the invention lies in a specific configurationof the reinforcement built up from the filaments, namely, in ametalization of the individual filaments.

For this reason, the electrically conductive filaments are an integralcomponent of the textile fabric ply (plies) of the air spring flexiblemember. The electrically conductive filaments are made of the same basicmaterial as the other, that is, nonconducting filaments and are onlycoated with a conductive surface. For this reason, an identical, thatis, homogeneous expansion behavior results over the entire wall of theflexible member. And because the electrically conductive filaments arenot arranged in a separate plane, there results also no stiffening ofthe wall of the flexible member and therefore also no additionalharshness effect. The electrically conductive coating is alreadyundertaken during the manufacture of the filaments. A separate work stepduring manufacture of the flexible member wall is therefore not present.

The wall of the flexible member, which is provided with electricallyconductive filaments in accordance with the invention, defines the basisfor the solution of the diverse measuring tasks.

With the solution set forth in the patent claims, not only (as required)a measuring method is provided for determining the spring height.

In addition, the air pressure, which is present in the air springflexible member, and the temperature of the wall of the flexible membercan be determined. Furthermore, a measurement of the fiber expansion ispossible. Likewise, occurring or already occurred damage because ofstone impact, wear, et cetera, can be detected early. Furthermore, it ispossible to transmit electrical energy along the spring flexible memberand to heat the air spring flexible member. The structures according tothe invention of conductive fibers to build measuring resistances,measuring capacitors and thermal elements are integral components of thefabric in the air spring. In this way, separate measuring quantitytransformers for the solution of the particular measuring task areunnecessary.

The integrated sensors are based on similar conductor structures, which,depending upon the circuitry, solve different individual tasks: thus,the conductor strips for expansion measurement can also be used fordetecting damage on the outer wall of the flexible member. The sameapplies for the capacitors for measuring flexible member pressure. Thecapacitors can also be used for detecting damage caused by wear.

In two applications, the hardware for evaluating the measuring signalsis very similar: the alternating current bridge for the evaluation ofthe capacitances between the fabric plies is also suitable forevaluating the inductivity of the conductor loop for the heightmeasurement.

In detail:

Measurement of the Temperature in the Wall of the Flexible Member

Up to now, discrete temperature sensors (for example, thermoelements)have to be vulcanized in order to be able to determine the temperaturein the wall of the flexible member. Alternatively, the temperature canbe contactlessly measured from the outside with the aid of pyrometricmethods. All methods described are complex and therefore limited toindividual applications (for example, in the development of airsprings).

Measurement of the Fiber Expansion

The fiber expansion in the fabric of an air spring can not, up to now,be measured directly.

Early Detection of Damage

Wear-caused damage to air spring flexible members (which do not yet leadto air losses) can up to now only be determined via a visual check.Because this is very complex, the air springs are, as a rule, utilizedso long until they get noticed because of air loss.

Transmission of Electrical Energy Along the Spring Flexible Member

The air spring flexible member comprises nonconductors. Up to now,cables are necessary in order to supply electronic components in theroll-off piston with electrical energy.

Electrical Heating of Air Spring Flexible Members

At the present time no heatable air springs are known.

The following advantages are presented individually:

a) Measurement of the Flexible Member Pressure

The otherwise required connection point for a pressure sensor is notnecessary because of the integrated measuring quantity converter.

b) Measurement of the Temperature in the Wall of the Flexible Member

The integrated resistance paths and the thermal elements, which areformed from conductive fibers, replace external components for measuringtemperature. Furthermore, all series air springs can be equipped with atemperature measurement with the conductor structures according to theinvention in the fabric. The operational reliability of the springincreases because of the monitoring of the temperature in the rollinglobe which is especially subjected to mechanical load.

c) Measurement of the Fiber Expansion

By connecting the light conductive fibers to form expansion measuringstrips, it is now possible to directly measure the expansion of thefibers, which function as reinforcement, and are located within theflexible member wall.

d) Early Detection of Damage

The early detection of damage to an air spring increases the reliabilityof the vehicle. Furthermore, the detection of wear-caused damage isimportant in the service life experiments in the context of thedevelopment of air springs.

e) Transmission of Electrical Energy Along the Spring Flexible Member

Electrical components, which are disposed in the piston of the airspring system or on the axle, can be supplied with energy withoutexternal cables. The electrical energy can be fed via a plug on thecover plate and separate cables are unnecessary because the conductorsare an integral part of the air spring flexible member.

f) Electrical Heating of Air Spring Flexible Members

Air spring flexible members with elastomers on the basis of chloropreneare not suitable for applications at temperatures below −25° C. becausethe elastomer reaches the glass transition point. For lowertemperatures, natural rubber is therefore used. With an electricalheating of the spring flexible member with the aid of the conductivefibers in the fabric, the temperature at the outer flexible member wallcan be so controlled that it always lies above the glass transitionpoint of chloroprene. In this way, the area of application of thiselastomer material expands.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the drawingswherein:

FIG. 1 shows the schematic representation of an air spring inlongitudinal section;

FIG. 2 a shows the schematic representation of a crossed arrangement ofcord fabric plies in an air spring rolling-lobe flexible member (notshown here in the entirety);

FIG. 2 b shows section A-A of FIG. 2 a;

FIG. 3 shows a reinforcement filament which is coated with a thin metallayer;

FIG. 4 shows a reinforcement fiber comprising filaments with some of thefilaments being metallized;

FIG. 5 shows the section from a fabric ply wherein a specific number ofconventional fibers are replaced with conductive fibers;

FIG. 6 a shows a perspective view of the external fabric ply of an airspring flexible member;

FIG. 6 b shows, in plan, two conductor loops which are electricallyconnected to each other at the lower end of the spring and are arrangedin the fabric;

FIGS. 7 a/b show schematics of the areas covered by the conductivestrips, namely, FIG. 7 a in the compressed state and FIG. 7 b in therebound state;

FIG. 8 shows a block circuit diagram with an alternating current bridgecircuit of the conductor loops (shown in FIGS. 6 a/b and FIGS. 7 a/b)for determining the height;

FIG. 9 shows a crossover location of two fabric plies;

FIG. 10 shows an alternating current bridge circuit for determining thecapacity of two crossover locations;

FIGS. 11 a/b show schematics of an expansion measuring arrangement, thatis, FIG. 11 a shows layer 1 and FIG. 11 b shows layer 2;

FIG. 12 shows a block circuit diagram with a wheatstone bridge fortemperature determination; and,

FIG. 13 shows a block circuit diagram with a wheatstone bridge forevaluating the expansion measurement according to FIGS. 11 a/b.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The schematic of FIG. 1 shows the essential details of an air spring 2:two end members (4, 6), which are spaced variably from each other, thatis, a cover 4 and a roll-off piston 6. The air spring also includes arolling-lobe flexible member 8 clamped pressure tight between the cover4 and the roll-off piston 6.

With the aid of a level control system (not shown), the height (level)h, which is given in each case between cover 4 and roll-off piston 6,can be controlled by changing the air pressure p present in the airspring volume 10. The rolling lobe 12 of the flexible member 8 rolls onthe outer wall of the roll-off piston 6. The rolling-lobe flexiblemember 8 comprises an elastomeric material 14 and is reinforced by areinforcement 16.

The reinforcement 16 of the air spring 2, as a rule, comprises twocrossed-over cord fabric plies (16 a, 16 b) (FIG. 2 a) which are eachvulcanized into the elastomeric material 14 of the flexible member 8(FIG. 2 b).

The basic idea of the invention is to intersperse the fabric plies (16a, 16 b) of the textile reinforcement 16 with fibers 18 which havefilaments 20 coated with a thin metal layer 22 (0.6 μm to 0.7 μm) inorder to rake them electrically conductive (FIG. 3). To achieve moderateconductivity, only individual metallized filaments 20 a are worked intothe fiber 18. The conductivity of the fibers 18 increases with thenumber of metallized filaments 20 a (FIG. 4). Also, entire fibers 18 canbe metallized (metallized fiber 18 a, FIG. 5). The filaments 20according to the invent: on and the fibers 18 are made, for example, ofpolyamide PA 6.6 which is coated with nickel, copper and/or silver(metal layer 22).

In the manufacture of the fabric plies (16 a, 16 b), a specific numberof conventional fibers 18 is replaced by conductive fibers 18 a (FIG.5). The number and density of the electrically conductive fibers 18 aand their electrical conductivity is determined in accordance with thetask to be solved.

a) Measuring the Spring Height

In a fabric ply 16 a or 16 b or in both fabric plies (16 a, 16 b), anumber of highly conductive fibers 18 a is connected in parallel to forma conductor strip (24 a and/or 24 b). Two conductor strips 24 from thesame fabric ply 16 a or 16 b, which lie on the periphery opposite eachother, are connected electrically to each other (bridge 26) at the lowerend of the air flexible member and form a conductive loop 24 whoseconductivity L is essentially dependent upon the developed area A (FIGS.7 a/b) which increases substantially linearly with the instantaneousspring height. For the evaluation, the conductor loop 24 is placed as achanging element in an alternating-current bridge circuit 28 and issupplied with a high frequency current. With a second conductor loop 24,which is arranged on the periphery offset by 90°, the complete bridgecan be assembled (FIG. 8) whose sensitivity can be varied via afrequency f of the supply current.

b) Measurement of Flexible Member Pressure

In each of the two fabric plies (16 a, 16 b), several highly conductivefibers are connected in parallel to form conductive loops (24 a, 24 b)which function as an equi-potential area for a capacitive arrangement.The conductor strips (24 a, 24 b) of the two fabric plies (16 a, 16 b)are normally insulated with respect to each other by the elastomer 14.Where the strips (24 a, 24 b) of the two fabric plies (16 a, 16 b)cross, an electric capacitance C results therebetween whose value isdependent upon the crossover area A_(C) of the two strips (24 a, 24 b)(FIG. 9) and from their distance d to each other. The crossover areaA_(C) as well as the distance d between the fabric plies (16 a, 16 b)are dependent upon the fabric angle γ which becomes less with increasingpressure p, in the spring 2.C=∈ ₀·∈_(r) ·A _(C)(γ)/d(γ).

While the crossover area A_(C) becomes less because of the fabric angleγ, which becomes ever smaller with increasing pressure p, the thicknessof the wall of the flexible member, and therefore the distance d betweenthe fiber layers (16 a, 16 b), does not change uniformly with thepressure p.

In order to determine the pressure p, the capacitances C at each twocrossover locations 30 above the rolling lobe 12 (FIG. 1) are evaluatedwith the aid of an alternating-current bridge circuit 28 (FIG. 10). Thesensitivity of the bridge circuit can be changed via the work frequencyf.

c) Measurement of the Temperature in the Wall of the Flexible Member(Resistance Measuring Bridge)

The basis of this method are the conductor structures for measuring thefiber expansion (FIGS. 11 a/b). The difference comprises that the fibers18 are made conductive with the aid of different metals which exhibitdifferent temperature coefficients. The temperature dependent andexpansion dependent resistance paths are connected as a wheatstonemeasuring bridge 32 (FIG. 12) and in such a manner that the expansion ofthe fibers 18 in the two conductor strips mutually compensate while thedifferent temperature coefficients lead to the condition that theresistance changes Δ_(R) unbalance the bridge 32 because of thetemperature T and generate a corresponding output signal U_(R).

d) Measurement of the Fiber Expansion

The expansion or stretching of a fiber 18 in the fabric 16 is dependentupon the position of the particular measuring point on the fiber 18. Theexpansion is minimal at the connection to the piston 6 and increasesoutwardly over the rolling lobe 12. The fiber expansion is a maximum atthe outer wall of the flexible member above the rolling lobe 12.

At this location, individual conductive fibers 18 a of defined lengthare collected together in one of the two fabric plies 16 a or 16 b sothat they form an expansion measuring strip (FIGS. 11 a/b). Theexpansion-dependent resistance change of the strip is evaluated with theaid of a wheatstone bridge circuit 32 (FIG. 13).

e) Early Detection of Damage

One Ply

For the detection of damage of the spring flexible member 8, severalconductive strips are formed from several fibers 18 in the outer fabricply 16 a or 16 b and their total resistance R is monitored (FIGS. 11a/b). Individual filaments 20 or fibers 18 a in the conductive stripsare damaged or cut through because of damage such as stone impact orabrasion whereby the total resistance R of the strips increases.

Two Plies

Wear-caused damage often begins with the separation of the elastomer 14from a fabric ply 16 a or 16 b. In order to detect this damage,conductive strips are formed in both fabric plies (16 a, 16 b) (FIG. 9).The capacitance C between these strips is monitored. The capacitancechanges very greatly when the elastomer 14 separates from a fabric ply16 a or 16 b because the resulting dielectric number ∈_(r) therebyreduces drastically.

f) Transmission of Electrical Energy along the Spring Flexible Member

For the transmission of electrical energy to electronic components,which are located in the piston 6 of an air spring 2, several highlyconductive fibers 18 a in a fabric ply 16 a or 16 b are combined to therequired number of conductors (FIG. 6 a, without bridge).

g) Electrical Heating of the Air Spring Flexible Members

A moderate number of conductive fibers 18 a of the two fabric plies (16a, 16 b) are connected together to form heater resistors and aresupplied with electrical energy (FIG. 6 a) in order to transfer heat tothe wall of the flexible member. The heating of the flexible member canbe limited to the rolling lobe 12, which is especially mechanicallyloaded, in order to reduce the requirement as to electrical heatingpower.

A control of the heating power ensures that the temperature T of theouter wall of the flexible member 8 does not drop below a criticalvalue. For the control, a temperature sensor is required on the wall ofthe flexible member in principle. The heating fibers in the fabric areslightly warmer than the wall of the flexible member on the outsidebecause of the heat conductance through the elastomer 14 so that alsothe temperature T of the wall of the flexible member can be applied as acontrol quantity.

REFERENCE CHARACTER LIST

-   2 air spring-   4, 6 end members-   4 cover-   6 roll-off piston, piston-   8 rolling-lobe flexible member, air spring flexible member-   h height (level) of the air spring-   10 air spring volume-   p air pressure-   12 rolling lobe-   14 elastomeric material, elastomer-   16 reinforcement, fabric-   16 a, 16 b (cord) fabric ply (plies) of the reinforcement, fiber    layer(s)-   18 fiber, fibers, filament(s)-   18 a metallized fiber-   20 filament(s)-   20 a metallized filament-   22 metal layer, electrically conductive coating-   24 conductor loop-   24 a conductor strips, number of highly conductive fibers 18 a (in    fabric ply 16 a)-   24 b the same in fabric ply 16 b-   A area developed by conductor loop 24-   26 bridge-   L inductivity of the conductor loop 24-   28 alternating-current bridge circuit-   f frequency, work frequency-   C electrical capacitance between the fabric plies 16 a and 16 b in    the region of the crossover of the two conductor strips-   A_(C) crossover area-   d distance of the fabric layers from each other-   γ fabric angle-   ∈₀ electrical field constant (=8.85416·10⁻¹² F·m⁻¹)-   ∈_(r) dielectric number-   30 crossover location-   T temperature-   U_(T) thermal stress-   32 wheatstone measuring bridge-   ΔR resistance change-   U_(R) output signal-   R (total) resistance

1. An air spring comprising: a cover; a roll-off piston disposed inspaced relationship to said cover; a rolling-lobe flexible member madeof elastomeric material and clamped pressure tight between said coverand said roll-off piston; said flexible member containing areinforcement vulcanized into said elastomeric material; saidreinforcement including two cord fabric plies arranged in said flexiblemember so as to cross over at an angle (γ); each of said cord fabricplies including a plurality of fibers; each of said fibers including aplurality of elastomeric filaments; and, at least a number of thefilaments of at least selected ones of said fibers being coated with anelectrically conductive material so as to be electrically conductivewhile simultaneously retaining the same strength as the remainder ofsaid filaments which are not coated.
 2. The air spring of claim 1,wherein the electrically conductive filaments have a carrier and anelectrically conductive coating covering said carrier.
 3. The air springof claim 2, wherein the electrically conductive coating is a metal layerwhich is tightly joined to said carrier.
 4. The air spring of claim 3,wherein the metallized filaments are coated with at least one of thefollowing metals: nickel, copper and silver.
 5. The air spring of claim1, wherein said selected ones of said fibers are metallized.
 6. The airspring of claim 1, wherein a first plurality of said fibers include saidselected ones of said fibers and a second plurality of said fibers aremetallized fibers; and, the ratio of said second plurality of fibers andsaid first plurality of fibers and the conductive value of said secondplurality of fibers is dependent on the current to be conducted thereby.7. The air spring of claim 1, wherein: in one fabric ply or in bothfabric plies, a number of highly conductive fibers are connected inparallel at the beginning and at the end of the particular fabric plyand in each case, form conductive strips; and, two of these conductivestrips of a fabric ply, which lie opposite each other at the periphery,are electrically connected to each other at the end of the air springflexible member and thereby form, in each case, a conductor loop.
 8. Theair spring of claim 1, wherein several highly conductive fibers of eachof said fabric plies form a conductive strip by being connected inparallel; the strips of the two fabric plies are insulated with respectto each other by the elastomer and form an electric capacitance (C) at acrossover location of said fabric plies whereat a crossover area (A_(C))is defined; the value of the electric capacitance is dependent upon saidcrossover area (A_(C)) of the two strips and on their distance (d) toeach other; and, said crossover area (A_(C)), in turn, is a function ofthe fabric angle (γ).
 9. The air spring of claim 8, wherein thecapacitance (C) of each two crossover locations, which are disposedabove the rolling lobe, are respective capacitive impedances of analternating-current bridge circuit.
 10. The air spring of claim 1,wherein said selected ones of said fibers are made conductive withrespectively different metals having respectively different temperaturecoefficients.
 11. The air spring of claim 1, wherein several conductivefibers in one fabric ply or in both fabric plies form several conductivestrips for the purpose of monitoring the total resistance (R) thereof.12. The air spring of claim 11, wherein several conductive fibers formconductive strips in both fabric plies; and, the strips form acapacitance (C) at their crossover locations and the magnitude thereofchanges in a measurable manner when there is a separation of a fabricply.
 13. The air spring of claim 1, wherein several ones of those fibershaving electrically conductive filaments are combined in a fabric plyfor the purpose of transmitting electrical energy or electrical signalsto at least one electronic component.
 14. The air spring of claim 1,wherein several ones of said fibers of both fabric plies have moderatelyconductive filaments and are connected to each other to form heatingresistors.
 15. The air spring of claim 14, wherein the rolling lobe ofsaid flexible member is subjected to mechanical load; and, said severalones of said fibers are limited to said rolling lobe.
 16. The air springof claim 15, wherein said air spring further comprises a thermal sensorfor detecting a critical limit value; and, a control for controllingsaid several ones of said fibers by considering said critical limitvalue.
 17. An air spring comprising: a cover; a roll-off piston disposedin spaced relationship to said cover; a rolling-lobe flexible membermade of elastomeric material and clamped pressure tight between saidcover and said roll-off piston; said flexible member containing areinforcement vulcanized into said elastomeric material; saidreinforcement including two cord fabric plies arranged in said flexiblemember so as to cross over at an angle (γ); each of said cord fabricplies including a plurality of fibers; each of said fibers including aplurality of filaments; at least a number of the filaments of at leastselected ones of said fibers being electrically conductive; in onefabric ply or in both fabric plies, a number of highly conductive fibersbeing connected in parallel at the beginning and at the end of theparticular fabric ply and in each case, forming conductive strips; twoof these conductive strips of a fabric ply, which lie opposite eachother at the periphery, being electrically connected to each other atthe end of the air spring flexible member and thereby forming, in eachcase, a conductor loop; and, the conductor loops of the two fabric pliesforming respective elements in corresponding branches of analternating-current bridge circuit.
 18. The air spring of claim 17,wherein an operating frequency (f) of the alternating-current voltage,which is applied to the alternating-current bridge circuit, is variable.19. An air spring comprising: a cover; a roll-off piston disposed inspaced relationship to said cover; a rolling-lobe flexible member madeof elastomeric material and clamped pressure tight between said coverand said roll-off piston; said flexible member containing areinforcement vulcanized into said elastomeric material; saidreinforcement including two cord fabric plies arranged in said flexiblemember so as to cross over at an angle (γ); each of said cord fabricplies including a plurality of fibers; each of said fibers including aplurality of filaments; at least a number of the filaments of at leastselected ones of said fibers being electrically conductive; severalhighly conductive fibers of each of said fabric plies forming aconductive strip by being connected in parallel; the strips of the twofabric plies being insulated with respect to each other by the elastomerand forming an electric capacitance (C) at a crossover location of saidfabric plies whereat a crossover area (A_(C)) is defined; the value ofthe electric capacitance being dependent upon said crossover area(A_(C)) of the two strips and on their distance (d) to each other; and,said crossover area (A_(C)), in turn, being a function of the fabricangle (γ); the capacitance (C) of each two crossover locations, whichare disposed above the rolling lobe, being respective capacitiveimpedances of an alternating-current bridge circuit; and, an alternatingvoltage having a selectable operating frequency (f) being applied tosaid alternating-current bridge circuit; and, said alternating-currentbridge circuit having a sensitivity which can be changed by selectingsaid operating frequency (f).
 20. An air spring comprising: a cover; aroll-off piston disposed in spaced relationship to said cover; arolling-lobe flexible member made of elastomeric material and clampedpressure tight between said cover and said roll-off piston; saidflexible member containing a reinforcement vulcanized into saidelastomeric material; said reinforcement including two cord fabric pliesarranged in said flexible member so as to cross over at an angle (γ);each of said cord fabric plies including a plurality of fibers; each ofsaid fibers including a plurality of filaments; at least a number of thefilaments of at least selected ones of said fibers being electricallyconductive; said selected ones of said fibers being made conductive withrespectively different metals having respectively different temperaturecoefficients; and, conductive fibers of each of said fabric pliesforming a conductive strip; the conductive fibers defining temperaturedependent and expansion dependent resistance paths which form elementsof a wheatstone measuring bridge in such a manner that the expansion ofthe fibers mutually compensate in the conductive strips; whereas, saiddifferent temperature coefficients have, as a consequence, atemperature-dependent unbalance with a corresponding output signal. 21.An air spring comprising: a cover; a roll-off piston disposed in spacedrelationship to said cover; a rolling-lobe flexible member made ofelastomeric material and clamped pressure tight between said cover andsaid roll-off piston; said flexible member containing a reinforcementvulcanized into said elastomeric material; said reinforcement includingtwo cord fabric plies arranged in said flexible member so as to crossover at an angle (γ); each of said cord fabric plies including aplurality of fibers; each of said fibers including a plurality offilaments; at least a number of the filaments of at least selected onesof said fibers being electrically conductive; and, in one fabric ply orin both fabric plies, individual fibers of defined length being combinedto define an expansion measuring strip and forming an element of awheatstone bridge circuit.